In accordance with a method known from EP-B1-0 253 760, a radially adjustable retainer, which is adapted to be driven in the circumferential direction of the stationary storage drum, is used for exactly dimensioning the length of the weft yarn and for decelerating the weft yarn at the end of the weft insertion process. During a weft insertion process, the retainer, which stands still in an engaged condition at a predetermined circumferential position of the storage drum, is disengaged so that, when the weft yarn is withdrawn, the withdrawal point will circulate at a rapidly increasing speed. In the course of the weft insertion process, the disengaged retainer is accelerated in the direction of circulation to a speed corresponding approximately to the speed at which the withdrawal point circulates. The withdrawal point will, however, first pass below the disengaged retainer. Then the retainer is engaged. The weft yarn moves into contact with the retainer at the withdrawal point, whereupon it will be decelerated by the engaged retainer until it is standing still at a new predetermined circumferential position. The retainer, which is included in a traveller together with an actuating magnet, has a comparatively big mass, and this will cause problems during acceleration and deceleration. It is difficult to activate the actuating magnet at the correct moment in the course of the rotary movement of the retainer. In view of the big mass which has to be decelerated, it is difficult to bring the engaged retainer to a standstill so that the deceleration process will be comparatively long. This will have a negative influence on the process of weft insertion in the weaving machine, since the deceleration phase lasts comparatively long.
A method known from JP 85-077 054 (60-28 552) comprises the steps of rotating the radially adjustable retainer in the circumferential direction of the storage drum as soon as said retainer has been disengaged so as to exactly dimension the length of the weft yarn. At the new position, the retainer is reengaged after the last admissible passage of the withdrawal point so that the weft yarn will reliably be caught. During the weft insertion process, the retainer does not influence the weft yarn withdrawal movement. At the end of the weft insertion process, the weft yarn is stopped abruptly. The storage drum is stationary.
In accordance with a method known from EP-A1-0 80 692, the storage drum as well as the retainer are rotatingly driven. The retainer is additionally adapted to be moved between an engaged position at which it blocks the circulatory path of the withdrawal point and a disengaged position.
In accordance with a method known from EP-A1-02 26 930 and used for feeding a yarn to a flat knitting machine, a changeover is effected between a positive feeding operation and free feeding of the yarn. A stationary storage drum has associated therewith a rotatable yarn guiding member, which is adapted to be driven such that it carries out a rotational movement about the storage drum axis and which is constructed in such a way that the withdrawal point of the yarn can overtake the yarn guiding member under certain pre-conditions (free feeding), whereas in the positive feeding mode the circumferential speed of the yarn guiding member determines the quantity of yarn fed per unit time.
In accordance with a method known from EP-A-0 477 877, the weft yarn, which is measured by a measuring prewinder by means of a radially adjustable retainer, is fed by an independently driven positive feed mechanism of the picking device, said positive feed mechanism being arranged subsequent to the storage drum. The positive feed operation is, however, discontinued during the weft insertion process so that the picking device continues to feed the weft yarn until it is standing still.
It is the object of the present invention to provide a method as well as a device of the type mentioned at the beginning by means of which the process of weft insertion can, on the one hand, be optimized as far as the weaving machine is concerned and, on the other hand, be carried out as gently as possible as far as the weft yarn is concerned; the present invention even aims at achieving a process of weft insertion which is adapted to be modulated from one weft insertion to the next. For jet weaving machines, it is the object of the present invention to make the weft yarn length exactly dimensionable, and in the case of shuttleless weaving machines, projectile or gripper weaving machines it is the object of the present invention to adapt the process of weft yarn withdrawal to an optimized weft insertion process.